Dual-pane thermal window with liquid crystal shade

ABSTRACT

A dual-pane thermal window unit comprises two non-intersecting or, preferably, substantially parallel, spaced window panes, mounted in a window frame, a first of the panes having affixed thereto a first wall of an electro-optical liquid crystal cell providing a selected light transmittance, and a second of said panes delimiting, with a second wall of said cell, a space providing a thermal break. Each of the first and second walls comprises an electrically conductive film composed of plastic and having sufficient supporting strength to maintain the structural integrity of the cell. The window is light weight, economical to manufacture and efficient and reliable in operation.

This application is a division of application Ser. No. 066,299 filed6-25-87.

BACKGROUND OF THE INVENTION

This invention relates to the use of a liquid crystal material toselectively control light transmission through a transparent orsemitransparent panel, and more particularly to the use of a liquidcrystal material to provide a window shade of adjustable transmittance,in combination with a dual-pane, heat-insulating window panel(thermal-pane window).

Thermal-pane windows conventionally make use of spaced dual panes toprovide a thermal barrier restricting heat conduction between theoutside and the inside of a building and therefore tending to reduceheating and cooling costs. To further reduce cooling costs, windowshades or blinds are used to block out intense, direct rays of sunlight,since conventional windows, insulating or otherwise, have little effecton radiative heating. However, in using a conventional shade toeliminate solar glare, the view to the outside is blocked, which may beconsidered a visually unattractive result. U.S. Pat. No. 4,268,126discloses a three pane window unit that overcomes this limitationwithout sacrificing energy efficiency by providing an electro-opticalshade as an integral part of a thermal pane window. The room occupantmay select the degree of light transmittance of the shade, thuseliminating glare and the adverse effect on cooling requirements fromdirect rays of the sun, while not blocking the view to the outside.

The present invention makes use of liquid crystal material to provideadjustable control of the intensity of light transmission through amulti-pane, thermal window unit. Generally, use of liquid crystalmaterials to selectively control light transmission through atransparent panel is known. Representative patents disclosing the natureof liquid crystal materials and their use are U.S. Pat. No 3,322,485 toWilliams, entitled "Electro-Optical Elements Utilizing an OrganicNematic Compound", U.S. Pat. No. 4,005,928 to Kmetz, entitled "NematicLiquid Crystal Displays for Low Voltage Direct Current Operation", andU.S. Pat. Nos. 4,268,126 and 4,456,335 to Mumford entitled "Thermal PaneWindow With Liquid Crystal Shade." New applications requiring improvedwindow constructions have necessitated efforts to develop thermal windowunits that are lighter and more easily fabricated.

SUMMARY OF THE INVENTION

The present invention provides an electro-optical shade of adjustablelight transmittance as an integral part of a dual-pane thermal windowunit. Advantageously, the window unit is resistant to radiative heatingand conductive heat transfer between the exterior and interior of abuilding. The window unit comprises two nonintersecting and, preferably,substantially parallel, spaced window panes, mounted in a window frame,a first of the panes having affixed thereto a first wall of anelectro-optical liquid crystal cell providing a selected lighttransmittance, and a second of said panes delimiting, with a second wallof said cell, a space providing a thermal break.

The term "electro-optical liquid crystal cell" as used hereinafter isintended to mean a volume of liquid crystal material between two closelyspaced electrodes, the liquid crystal material being electro-opticallyresponsive to an applied voltage between the electrodes, such that lighttransmittance through the liquid crystal material is selectabledepending upon the resulting electric field strength, current flow, orchange passed through the cell. Additionally, the "electro-opticalliquid crystal cell" can contain sealant layers, support layerscomprised, in one embodiment of the invention, of a plurality of wallsof transparent, electrically conductive film having sufficientsupporting strength to maintain the structural integrity of the cellwhen liquid crystal material is disposed between opposing faces of thewalls; binders; polarizer elements; and associated adhesives, asdiscussed hereinafter in more detail.

As used herein the term "pane" means a transparent or semitransparent,inorganic or organic material having mechanical rigidity and a thicknessgreater than 1 mm.

The term "electrically conductive film" as used herein means a layer orsequence of layers containing an electrically conductive layer withoutdifferentiation of the position of the conductive layer in a sequence oflayers. The electrically conductive layer can consist of a conductivefilm of uniform or of nonuniform thickness or of a sheet-like array ofsubstantially parallel wires.

The window unit may further comprise window frame means for securing themutual orientation of a plurality of transparent, nonintersecting or,preferably, substantially parallel, sequentially spaced panes and forsealingly isolating a space therebetween; a first transparent panemounted in the window frame means in a position toward an exteriorfacing side of said frame means; a second transparent pane,nonintersecting with and, preferably, substantially parallel to andspaced from said first pane, mounted in said frame means in a positiontoward an interior facing side of said frame means; a liquid crystalcell comprising a first wall composed of transparent, electricallyconductive film, a second wall composed of transparent, electricallyconductive film and liquid crystal material disposed between opposingfaces of said first and second walls, said liquid crystal material beingelectro-optically responsive and said electrically conductive filmhaving sufficient supporting strength to maintain the integrity of saidcell; said first wall of said cell being affixed to one of the opposingfaces of said first and second panes and said second wall of said celldelimiting with the other opposing face of said first and second panes aspace providing a thermal break; and electrical means for applying anelectric field between said conductive films and through said liquidcrystal material of a selected field strength at least sufficient tochange the optical transmission of said liquid crystal material.

The invention further provides a method for decreasing radiative heatingand conductive heat transfer between the exterior and the interior of abuilding, comprising the steps of: mounting within a window frame aplurality of spaced window panes, a first and second of said paneshaving opposing faces; affixing to one of the opposing faces a firstwall of a liquid crystal cell, said first wall being composed oftransparent, electrically conductive film and cooperating with a secondwall composed of transparent electrically conductive film to form acavity containing an electro-optically responsive liquid crystalmaterial providing a selected light transmittance; and delimitingbetween said second wall of said cell and the other of said opposingfaces of said panes a space providing a thermal break.

Advantageous structural features are provided by the method and means ofthis invention. The liquid crystal cell is readily produced as film onrolls for application to the sizeable area provided by either opposingface of the panes. Once applied, a thermal break is achieved withoutneed for more than two panes of glass. The size, weight and cost of thewindow unit is markedly reduced, manufacturing procedures are simplifiedand the reliability and operating efficiency of the unit are increased.

The liquid crystal material may be selected as one beingelectro-optically responsive over a substantially full range oftransmittance to an AC or DC field which is preferably at low voltage tominimize the risk of shock hazard in the event of breakage or electricalmalfunction.

The panes may be light polarizing to further reduce glare from directsunlight or to increase the efficiency of the electro-optical liquidcrystal cell. An inert gas may be injected into the space delimitedbetween the second wall of the cell and an opposing face of a pane, orthe space may be evacuated to the extent practical to enhance thermalconductivity break characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood and further advantages willbecome apparent when reference is made to the following detaileddescription of the preferred embodiment of the invention and theaccompanying drawings in which:

FIG. 1 is a perspective view of a dual paned window of the presentinvention in a typical frame; and

FIG. 2 is a cross-sectional view taken along line 2--2 of FIG. 1,showing a thermal barrier space between a wall of the liquid crystalcell and an opposing face of a pane;

FIG. 3 is a sectional view showing an alternative embodiment in which awall of the liquid crystal cell is attached to a fractional portion ofan opposing face of a pane;

FIG. 4 is a sectional view showing another embodiment wherein pluralliquid crystal cells are employed, a first of said cells being attachedto one of the opposing faces of a pane and to a wall of a second of saidcells;

FIG. 5 is a sectional view showing still another embodiment wherein aplurality of panes, a plurality of liquid crystal cells and a pluralityof thermal breaks are employed, each of the thermal breaks beingdelimited between a wall of each of the cells and a face of each of thepanes; and

FIG. 6 is a sectional view showing a further embodiment wherein aplurality of liquid crystal cells disposed in spacially separate fashionbetween two panes provide a plurality of thermal breaks

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring specifically to the drawings, in FIG. 1 there is shown awindow unit 1 having two nonintersecting and, preferably, substantiallyparallel, spaced transparent panes 6 and 8 mounted in a conventionalframe 5. A cross-sectional view taken along the line 2--2 in thedirection indicated by the arrows is shown in FIG. 2.

Transparent panes 6 and 8 are mounted in channels 4 of frame 5 with aconventional semi-rigid sealant 9, such as butyl rubber, so that thepanes are nonintersecting and, preferably substantially parallel andspaced. The sealant aids in securing the mutual orientation of the panesand seals or isolates the space between the panes. The window unit ismounted in a window opening of a wall structure so that pane 6 is theoutside pane and pane 8 is the inside pane. Panes 6 and 8 and the space10 constitute the thermal-pane portion of the embodiment wherein space10 provides a thermal barrier significantly restricting the conductionof heat through the window. Frame 5 is shown as being hollow, by way ofexample, to restrict peripheral heat conduction and may be an extrudedaluminum alloy. To enhance the thermal barrier effect, space 10 may beevacuated to the extend practical, or filled with an inert gas selectedfrom the group consisting of argon, nitrogen, dry air neon and mixturesthereof. Use of an inert gas, such as nitrogen, inside of the thermalpane can be usefully employed to prevent oxidative degradation of theliquid crystal, polarizer elements, and adhesive layer. Often inertgases, such as argon or neon can also be employed, but are lesspreferred from a cost viewpoint than nitrogen

Affixed to one of the opposing faces of panes 6 and by means of asuitable adhesive is a first wall 11 of a liquid crystal cell. A varietyof adhesives can be conveniently utilized. Preferably the adhesiveshould thoroughly wet the surface of the film sandwich, so as to ensureproper bonding and the elimination of spurious void spaces which canscatter light and interfere with sound mechanical adhesion. Also, theset adhesive is preferably colorless and either amorphous ormicrocrystalline with a crystallite size much smaller than thewavelength of light, so that negligible light scattering or absorptionof light occurs at the adhesive interface. Adhesives found especiallysuitable for this purpose are certain polyvinylacetate adhesives, orcyanoacrylate adhesives and the like. Wall 11 is composed of atransparent, electrically conductive film, such as tin oxide depositedon a transparent film composed of plastic such aspolymethylmethacrylate, polycarbonates and the like, and cooperates witha second wall 7 composed of transparent, electrically conductive filmhaving the composition of wall 11 to form a cavity containing a liquidcrystal material 14. Electrical leads 13 connect the first and secondwalls 11 and 7 (which constitute electrodes) to a variable voltagesupply 15. Liquid crystal material 14 fills substantially the entirevolume of the cavity. Typically, the distance between opposing faces ofwalls 11 and 7 is about 1 mill.

Electro-optical liquid crystal cells of the type described herein can beconveniently fabricated by a variety of techniques known in the art. Forexample, the cell can consist of two plastic sheet electrodes madeconductive by means of a tin oxide coating, separated by a melt orsolution-formed microdispersion of liquid crystal material, thepreparation of which is described by P. S. Drzaie, "Polymer DispersedNematic Liquid Crystal For Large Area Displays and Light Valves", J.Appl. Phys. 60(6) Sept. 15, 1986. Alternately, one electrode of theelectro-optical liquid crystal cells can result from a conductivecoating, such as tin oxide, applied to a glass pane. This conductingglass pane can then be coated by the above-described microdispersion ofliquid crystals in plastic binder which is then covered with aconductive plastic sheet to complete the cell. Generally, the liquidcrystal need not be included in a microdispersion but can rather bedeposited as a distinct layer between each of the above-describedelectro elements, as described in "Topics in Applied Physics", Vol. 40,"Display Devices", edited by J. I. Pankove, Springer-Verlag, New York(1980). That is to say, the liquid crystal material can be depositedbetween walls 7 and 11 transparent electrically conductive plastic film,which walls have sufficient supporting strength to maintain theintegrity of the cell during installation and operation of the windowunits as described hereinafter in more detail.

The nature of liquid crystal materials may be summarized as follows: Thethree common states of matter are the solid, liquid, and gas states inwhich the randomness of geometrical arrangement of the moleculesincreases from the solid to the liquid to the gas. The gas and theordinary liquid are both isotropic, having the same physical propertiesin all directions. Most solids are found to be crystalline; that is,their molecular units are arranged in a regular repeating geometricalpattern of lattice units and consequently are frequently anisotropic inthat their physical properties vary depending upon the direction ofmeasurement with respect to different crystal axes. Certain organicsolid compounds exhibit a peculiar behavior such that upon melting aturbid melt results that changes abruptly to clear isotropic liquid uponheating to a higher temperature. In this temperature range, thesecompounds are anisotropic with respect to transmission of light. Thus,the characteristics of these compounds are partly those of the isotropicliquid since they exhibit liquid flow and partly those of theanisotropic solid. Therefore, these materials are often called "liquidcrystals" or, more accurately, "crystalline liquids" and are sometimesclassified as a fourth state of matter referred to as the mesomorphicstate or mesophase, being a state or phase intermediate that of theanisotropic crystal and that of the isotropic liquid. There areessentially two major classes of liquid crystals, the "nematic" stateand the "smectic" state. The nematic liquid crystal materials generallyconsist of rod-shaped molecules that tend to align parallel to a commondirection resulting in anisotropy for many of the bulk properties. Whenthe alignment is uniform, the sample is optically clear. However, when acritical voltage is applied (typically 2 to 20 volts per mil) to theliquid crystal film, a critical current flow disrupts the uniformalignment causing scattering or refraction of incident light, termeddynamic scattering. As a consequence of this scattering, the lightintensity transmitted through the liquid crystal cell decreases. Thesmectic state is a more highly ordered state than the nematic state. Thesmectic state does not exhibit dynamic scattering in response to anelectric field. On the other hand, smectic materials are desirable inthat they exhibit a very low crystal-to-mesomorphic transitiontemperature and often exist in a mesomorphic state at room temperatures.However, mixtures of smectic and nematic materials may be produced whichare effective in dynamic scattering at room temperatures. Further,mixtures of crystalline liquids, such as cholesteric liquid crystals andthe like, can be used for purposes of tint or coloration. An ordinarynematic liquid crystal or smectic C phase can be converted into acholesteric by adding an optically active compound.

Liquid crystalline materials can be used in several different modes toeffectively control the transmission of light. These modes utilize (1)dynamic light scattering from the turbulant flow of liquid crystals inan applied electric field, (2) the development of changed birefringenceand resulting changes in rotation of light polarization as a consequenceof the application of an electric field, or (3) changes in lightabsorption as a consequence of the application of an electric field. Inthe first application mode, the window material assumes a white, hazyappearance as a consequence of the applied field. In the secondapplication mode, where the rotation of light due to opticalbirefringence is utilized, the liquid crystal is sandwiched between twooptically polarizing films. These films are crossed with respect to thepolarization of light which is transmitted. Then the development of achanged birefringence of the liquid crystalline layer (as a result ofthe application of an electric field) changes the rotation angle of theplane of polarization of light in the layer, so that light transmissionoccurs through the liquid crystal-polarizer ensemble. Alternately, thedesign changes known in the art permit operation in a mode where lighttransmission through two polarizers decreases when voltage is applied.In the third general type of application mode, the effect of appliedelectric field is to change the absorption characteristics of the liquidcrystal layer. This can be done in several different ways. First, theliquid crystal layer can be composed of a cholesteric liquid crystal.The light absorption characteristics of such layers depends upon thepitch of the helical structure of the molecular arrays, which is changedby the application of an electric field. Alternately, in a secondembodiment, the change in absorption properties of the liquid crystalupon the application of an electric field results from the orientationof monomeric or polymeric dye molecules which are incorporated in theliquid crystal phase. The requirements for these dye molecules are (1)that they must be dichroic, that is to say, they must selectively absorblight only of a specified polarization relative to the molecular axes,(2) that they intimately mix in the liquid crystal, and (3) that theyhave sufficiently anisotropic shape that this intimate mixing results inpreferential orientation of the dye molecules with respect to theoriented liquid crystal film. A detailed discussion of liquid crystalshutters is set forth in chapter 4 of "Topics in Applied Physics",Supra, pp. 151-180.

The voltage supply to the electrodes of the present invention ispreferably taken from ordinary AC household supply for purposes ofsimplicity. Further, it is preferred to reduce the voltage by atransformer to a voltage less than about 15 to 20 volts to minimize therisk of shock hazard to the user in the event of breakage ormalfunction. Depending upon the choice of designs for the liquid crystalcell, rectification of this AC voltage might be necessary for celloperation. A fuse may be inserted on the secondary side of thetransformer as an additional safety feature. The transformer isadjustable from zero volts up to the voltage in the particular liquidcrystal material where minimum transmittance (or maximum transmittance,for inverted operation modes) is achieved, preferably less than roughly20 volts. Alternatively, if the particular liquid crystal material isresponsive to a low voltage DC supply, then the control unit may bebattery powered.

Various liquid crystal materials are commercially available, andtherefore it is not the intent to limit the present invention to the useof any particular one so long as certain general requirements are met.The liquid crystal material should have an operating temperature rangeat least coextensive with the range of temperature usually experiencedin a habitable building allowing for temperature extremes near windows.The liquid crystal material should be colorless for purposes ofappearance of the window unit. Alternatively, mixtures of liquid crystalmaterials such as spontaneously twisted nematics and cholesteric liquidcrystal substances, may be used to change the color of the shade forpurposes of tinting or for decorative poses. The liquid crystal materialshould preferably be responsive to a low voltage AC power supply (eitherwith or without rectification to DC). For illustrative purposes only,examples of liquid crystal materials generally meeting theserequirements are shown in the above cited U.S. patents and are herebyincorporated by reference. Further examples are shown in U.S. Pat. Nos.3,829,491 and 3,809,656, the teachings of which are hereby incorporatedby reference thereto, wherein one such representative nematic materialis N-(p-methoxybenzylidene)-p-n-butyl aniline. Other liquid crystalmaterials which have been found suitable for this purpose are side-chainliquid crystal polymers such as those described in Chapter 2 of"Polymeric Liquid Crystals", edited by A. Cifferri et al, AccademicPress, New York, (1982) and in G. W. Gray et al, "Smectic LiquidCrystals", Leonard Hill, London, (1984). The Gray et al publication alsoprovides key references for discotic phase liquid crystals, consistingof flat disc-shaped molecules, which can also be employed as the liquidcrystal elements of the present electro-optical cells.

In use, the window unit 1 may be mounted in hinged and thermally sealedfashion over the inside of an existing window. The window unit ispreferably placed so that the thermal barrier (pane 6) is towards theoutside and (pane 8) to which the cell containing liquid crystalmaterial is affixed is towards the inside. Since many of the liquidcrystal compositions and optical polarizer elements degrade uponexposure to ultraviolet light found in the solar spectrum and sinceglass is a good absorber of ultraviolet light, incorporation of theliquid crystal element on the building interior inside of a glass paneis preferred. Also, such incorporation protects the liquid crystal filmand associated electrical, adhesive, and optical elements from theeffects of weathering. The incorporation of the shutter element (liquidcrystal film and associated electrical, adhesive, and optical elements)on the building-interior side of the thermal pane has the advantage ofprotecting this element from temperature extremes and the effects ofextreme temperature cycling. Incorporation of the shutter element insidethe thermal pane has the additional advantage of protecting this elementfrom mechanical changes during window cleaning (abrasion) and from thedamaging effects of ultraviolet light generated by fluorescent lights.

Alternatively, or for new construction, the window unit 1 may be usedalone without a conventional window. Thus, the user is afforded in oneunit a storm window and an electro-optical shade having variable lighttransmittance, selectable at the user's option.

Further features of convenience may be added. For automatic operation,an optical sensor may be included in the circuitry so that the opacityof the shade is controlled automatically according to the intensity oflight impinging on the outside of the window. Opacity may be furthercontrolled automatically in accordance with the time of day, the outsidetemperature, or status activity of air conditioning and heating units toprovide an integrated climate control within the room. To facilitatemaintenance, access conduits and valves may be provided for space 10 forpurge and/or refill. To reduce glare particularly when the unit isoperated to the full transmittance mode and to reduce opacityrequirements of the liquid crystal cell, panes 6 and/or 8 may be lightpolarizing. Use of a metal oxide coating such as the tin oxide coatingapplied to the plastic film of walls 11 and 7 additionally serves toreduce transmission of long wavelength, infrared radiation such as thatemitted by warm indoor objects. Heat loss due to radiation transmissionthrough the window from interior portions of the building is therebyreduced. The plastic film itself can function as an insulator to furtherreduce heat loss from the building. A mixture of crystalline liquids maybe employed to tint or change the color of the shade. The panes can benonplanar, as in embodiments used as skylights, wherein the panes areconvex. Interpane spacing can be variable. In a further embodiment ofthe invention, shown in FIG. 3, the shade may be affixed to a fractionalportion of one or more opposing faces and, or plural shades may be soaffixed, or affixed to substantially the entire surface areas of theopposing faces, or affixed to each other, as shown in FIG. 4, to providemultiple tinting or coloration of panes 6 and/or 8. The fractionalportion of a pane to which the shade is affixed can range from 5 to 95percent and preferably from 40 to 80 percent thereof. The fractionalportions of panes 6 and 8 so shaded are typically arranged in acomplementary fashion to maximize the area shaded during operation ofthe window unit in the minimum transmittance mode. Fractional shading ofpanes 6 and/or 8 may be used to produce multitint or multicolor borders,script, artistic renditions as found in stained glass windows, and thelike.

In still another embodiment, shown in FIG. 5, the window unit cancomprise a plurality of nonintersecting or, preferably, substantiallyparallel, spaced window panes 6, 8, 16; a plurality of liquid crystalcells 14, 14', each of the cells being affixed to a different one of thepanes; and a plurality of thermal breaks 10, 10', each of the thermalbreaks being a space delimited between a wall of each of the cells and aface of each of the panes. The number of thermal breaks afforded by thisembodiment of the window unit is generally two; one less than the numberof panes employed. It will be understood, however, that window unitconstructions can be readily produced wherein more than two thermalbreaks are achieved, as in the order of up to ten or more thermal breaksper unit. The provision of multiple thermal breaks affords an energysaving feature that renders the unit more cost effective than windowunits having a single thermal break.

The window panes 6 and/or 8 can be glass or plastic. In constructions ofthe window unit wherein one or more window panes are plastic, the liquidcrystal composition can be included as a microdispersion inside theplastic of the pane. This can be accomplished in several ways. Forexample, a polymer sheet containing liquid crystal globules can bedeposited from an aqueous phase emulsion of a water soluble polymer(such as polyvinyl alcohol), or a colloidal suspension of a waterinsoluble polymer (i.e., a latex emulsion), or a polymer melt.Alternately, it is possible to microencapsulate the liquid crystaldroplets within a molten or solubilized polymer. Thereafter a plasticsheet is produced by conventional melt-forming or solution-formingmethods. The resultant plastic sheets containing liquid crystalglobules, are then coated on both sides with a transparent electricalconductor such as tin oxide. Such methods for incorporation of liquidcrystals into polymer film are described in detail by P. S. Drzaie,"Polymer Dispersed Nematic Liquid Crystal For Large Area Displays andLight Valves", J. Appl. Phys. 60(6) Sept. 15, 1986. Electro-opticalliquid crystal cells wherein plastic sheets containing liquid crystalglobules are coated on both sides with a transparent electricalconductor such as tin oxide are readily produced using conventionalmanufacturing procedures and are, for this reason, preferred.Advantageously, with this construction of the pane, the liquid crystalelement is protected from damaging effects of oxygen and oxidizingagents found as pollutants in the atmosphere. Polymers consideredespecially useful for this purpose include polymethylmethacrylate andpolycarbonates.

It is often advantageous to utilize more than one liquid crystal sheetin the window ensemble. As a result of using more than one liquidcrystal sheet, greater range of optical switching capability can beachieved. Also, since the color change upon the application of anelectric field can be different for these different sheets, the abilityto selectively apply voltage to either one or both of these sheetsprovides the ability to change the color of the window over a wide rangeof different colors. As an additional advantage, the use of two sheetspermits selective area control over the transmissive properties of thewindow. For example, one of the sheets can extend over substantially theentire visible surface area of the window, while the second sheet canextend over only a fixed fraction thereof, such fraction beingpreferably about 20% to 50% of the visible surface area of the window.The effect then is to have dual control, first over the opacity of thetotal window and second over the opacity of the fractional portion ofthe window's visible surface area. In this manner, there is achieved adynamic selective-area tinting, such as typically found in a static casefor many automotive front windows. Several different liquid crystalsheets (which can be independently controlled) can be placed on top ofone another, so as to eliminate one of the conductive layers which wouldotherwise be necessary if these sheets where spacially well separated.Alternatively, as shown in FIG. 6, a plurality of liquid crystal cells14, 14' can be disposed in a spacially separate fashion between panes 6and 8 to provide a plurality of thermal breaks 10, 10' therebetween.

Flexibility in the change in color during the switching process can beachieved in a variety of different ways depending upon the applicationmode of the liquid crystals. If a change in opacity is desired without amajor change in the spectral distribution of transmitted and/orreflected light, it is useful to employ the change in dynamic scatteringof liquid crystals upon the application of an electric field.Alternatively, for those application modes which utilize crossedpolarizers, this same objective is readily achieved by means ofpolarizers which have the same optical density throughout the visiblespectral range and by the use of liquid crystal compositions, thebirefringence of which is not strongly dependent upon wavelength.Dramatic color changes are obtained upon optical switching by means ofseveral embodiments. A first embodiment comprises use of shuttermaterial composed of cholesteric liquid crystals, the latter materialsbeing typically highly colored. In a second embodiment there is utilizedas shutter material a nematic or smectic liquid crystal which has littlecolor, but is used in conjunction with crossed polarizers adapted toabsorb nonuniformly in the visible spectral range. Methods of makingpolarizers which result in conjugated hydrocarbons, such as polyenes,typically produce colored polarizers suitable for this embodiment. Bycontrolling conjugation length of these dichroic molecules, the color atwhich major absorption occurs can be systematically shifted. In yetanother embodiment, the color of the liquid crystal, or window shuttermaterial, is varied in inactivated and activated states by incorporatingtherein one or more types of dye molecules which have thecharacteristics mentioned hereinabove.

A gradient in absorption characteristic is often of functional utility,it generally being desired that the uppermost part of the window be moststrongly absorbent. Such a gradient can be readily accomplished in avariety of different ways. Generally, for shutters employing liquidcrystals in conjunction with polarizers, the polarizing films areconstructed by nonuniform chemical, thermal, or mechanical treatments sothat there is a gradient in dichroism of the polarizer sheets.Alternatively, a gradient in optical shutter characteristics can beachieved by use of a liquid crystal layer which has a nonuniformthickness, most preferably being wedgelike with the thickest part of thewedge being at the top of the window. As another possibility forobtaining nonuniform optical switching of the liquid crystal sheet, theelectric field can be applied nonuniformally over the window area byinsulating the electrodes for one part of the window area from those foranother.

Generally, in the design of the liquid crystal cells the conductivelayers and the electrical connects to the layers are made so thatequipotential surfaces result. In such cases, via the use ofequipotential connects and the use of materials for the conductinglayers which have high volumetric conductivities and large thicknesses,uniform electrical fields can be developed across uniform thicknesses ofliquid crystal compositions. For cases where highly conductive layersand equipotential contacts are used, such designs provide that there isno significant voltage drop across the conductive layers, no significantresistive heating in the layers, and uniform switching behavior of theliquid crystal film. However, in certain cases it is desireable thateither there is resistive heating within the conductive layers or thatthese conductive layers are not equipotential surfaces. For exampleresistive heating within these layers can be used to maintain liquidcrystal compositions within the functional temperature range of thesecompositions and to eliminate window fogging or frost formation. Also,voltage drops due to the finite resistance of the conductive layers canbe used to selectively vary the response of the liquid crystals over theoptical shutter area. This can be accomplished by applying voltage as agradient across at least one of the walls of conductive film. Morespecifically, this can be done by applying a voltage V₁₁ to a contactstrip at the bottom and a voltage V₁₂ to a contact strip at the top ofone side of the electrochemical cell and a votage V₂₁ to a contact stripat the bottom and a voltage V₂₂ to a contact strip at the top for thereverse side of the liquid crystal cell. As a consequence of this choiceof voltage distribution for opposing sides and opposing ends of thewalls of the liquid crystal cell, the voltage supplied to the liquidcrystal layer varies from V₁₁ -V₂₁ at the bottom of the cell to V₁₂ -V₂₂at the top of the cell. Consequently, a gradient can be achieved acrossthe window surface in the switching characteristics of the liquidcrystal cell. The nature of the voltage drop in each conductive surfacean be conveniently further modified by varying the thickness of theconductor layer across this surface. The difference in V₁₁ and V₁₂provides the resistive heating of the first conductive layer. Thedifference in V₂₁ and V₂₂ provides the resistive heating of the secondconductive layer. Since V₁₁, V₁₂, V₂₁ and V₂₂ are variable independentlyand since the thicknesses of the associated conductive layers are alsovariable, as well as the voltage applied on the left hand and the righthand side of the conductive sheets in the liquid crystal cell, one canconveniently vary both the degree of resistive heating in eachconductive layer as well as the spatial nature of the voltage applied tothe liquid crystal layer. Hence, convenient control of temperature andthe areal switching behavior of the liquid crystal sheets areindependently achievable.

The polarizer sheets appointed for use in the fabrication of a liquidcrystal shutter need not be in close proximity to the liquid crystalsheet. For example, one of the polarizer sheets could be provided with arolled configuration such as that found in a window shade or a stackedconfiguration such as that found in slat blinds.

An alternative to the use of liquid crystal shutters that are voltage orcurrent controlled, would comprise utilization of liquid crystalcompositions wherein switching is responsive to integrated current flow.For example, liquid crystal phases are employed which become reversiblyisotropic as a consequence of reversible electrochemical reactions, suchas found in acid-salt chemistry.

A variety of transparent conductors, such as SnO₂, In₂ O₃ and Cd₂ SnO₄and the like, can be used as voltage plates for the liquid crystalswitch. Examples of commercial compositions for such conductors aretransparent metal oxides made by Deposition Technology andSierracin/Intrex using sputtering techniques involving reactive gases incombination with metal targets. Leybold-Heraeus also offers commerciallya metal/metal oxide coating called TCC 2000 which is sufficientlytransparent and conductive for the present application.

Having thus described the invention in rather full detail it well beunderstood that these details need not be strictly adhered to but thatfurther changes and modifications may suggest themselves to one havingordinary skill in the art, all falling within the scope of the inventionas defined by the subjoined claims.

We claim:
 1. A method for decreasing radiative and conductive heattransfer between the exterior and the interior of a building, comprisingthe steps of:(a) mounting within a window frame a plurality of spacedwindow panes; (b) affixing to said panes a plurality of liquid crystalcells, each of said cells being affixed to a different one of saidpanes; and (c) delimiting within said window frame a plurality of spacesproviding a plurality of thermal breaks, each of said spaces beingdelimited between a wall of each of said cells and a face of each ofsaid panes.
 2. A thermal-pane window unit resistant to radiative heattransfer as well as conductive heat transfer, comprising:(a) a pluralityof substantially parallel, spaced window panes, mounted in a windowframe; (b) a plurality of liquid crystal cells, each of said cells beingaffixed to a different one of said panes; and (c) a plurality of thermalbreaks, each of said thermal breaks being a space delimited between awall of each of said cells and a face of each of said panes.